1. Field of the Invention
The invention generally relates to a sealing device for turbine engines. Specifically, the invention is directed to an annular-shaped windback disposed about a rotatable runner with a plurality of angled or inclined baffles along the outer diameter surface of the runner so that the baffles are interposed between the runner and the windback. The baffles and windback are positioned between a lubricant sump and a shaft seal so as to prevent lubrication fluid from reaching the seal and entering the gas side of a turbine engine.
2. Background
A lubricant sump is frequently required along a rotatable shaft to properly lubricate shaft bearings and the like. It is frequently desirable to isolate the lubricant sump from remaining media in which the shaft is immersed, as well as to assure that the lubricant remains in the sump so that lubrication of the bearings is continuous and the lubricant does not contaminate any other media in which the shaft is immersed.
In gas turbine applications, the lubricant sump is typically isolated from the remainder of the gas turbine housing by the sump housing and a shaft seal. The remainder of the gas turbine housing, or a compartment adjacent thereto, is typically filled with gases at temperatures and pressures higher than those desired in the lubricant sump. It is often important to prevent lubricant from mixing with gas contained in such high temperature gas and/or pressurized compartments. For example, when an oil lubricant is used, mixing between the oil and the gas could result in formation of oil coke, which is a byproduct of oil heated to an elevated temperature and which chemically alters the oil and hence is detrimental to the gas turbine. Oil coke can foul seal surfaces and prevent proper bearing lubrication. Accordingly, it is important in many applications that the lubricant be isolated within a lubricant sump.
To accomplish such sump isolation, a shaft seal is normally positioned adjacent to the sump and around a rotatable shaft passing therethrough. While the sump is typically at a lower pressure than the remainder of the turbine, research has shown that pressure differentials within the sump are such that lubricants held within the sump are encouraged to travel along the rotating shaft toward the seal. These pressure differentials, along with shaft windage, tend to move the lubricant out of the sump and along the shaft, a phenomenon often termed lubricant splash, which results in loss of lubricant from the lubricant sump.
Some sealing means, such as the shaft seal, are normally provided to at least minimize if not prevent lubricant splash and lubricant loss. However, the shaft seal, which typically is a segmented carbon rubbing seal, may have the carbon sealing faces contaminated by the lubricant if the lubricant reaches the seal. Such contamination of the seal can compromise the integrity of the seal and reduce the efficiency with which the seal isolates the lubricant within the lubricant sump.
To prevent lubricant from reaching the carbon sealing faces or other parts of the seal, a windback device is typically interposed along the shaft, between the seal and the lubrication sump. The windback device normally has an annular collar-like member, receiving the shaft in the opening of the collar-like member, and a screw pitch or thread in the radially inward surface of the collar-like member that faces the shaft. The windback device is typically secured to the housing to prevent lubricant leakage from the sump to the remainder of the housing except along the shaft. The shaft seal is normally interposed between the windback device and the shaft, with the seal located along the shaft, on the side of the windback device that is remote from the sump in order to prevent lubricant leakage along the shaft.
Since a windback device is typically in close proximity to the associated shaft, yet not so close as to expect to create a pressure drop along the shaft, initial conventional wisdom was that pressure along the opening in the windback device, namely at positions in the windback device opening that were adjacent to the rotating shaft, would be essentially constant along the length of the shaft within the windback device. As a result of this assumption, conventional wisdom has been that any lubricant splash moving along the shaft of the windback device would likely flow back toward the sump along the threaded inner surface of the windback collar.
Contrary to conventional wisdom, pressure measurements taken along the axial length of the windback device have shown a pressure drop between the sump and the end of the windback device remote from the sump. Measured pressure at the sump end of the windback opening was actually greater than measured pressure at the seal end of the windback opening. Thus, the pressure differential encourages lubricant to move away from the sump along the shaft to other interior portions of the shaft housing.
In applications such as gas turbines, the pressure of the lubricant medium in regions other than in the sump is higher than the pressure within the sump. Thus, one would expect that the lubricant pressure in the sump, adjacent to the seal, would be higher than the lubricant pressure in the remainder of the sump. However experiments have shown that, even in such applications, the pressure in the sump adjacent to the seal was lower than the pressure in the remainder of the sump, thus causing additional lubricant flow along the shaft towards the seal. Because of this unexpected pressure profile, windback devices prior to the aforementioned research failed to adequately prevent the flow of lubricant toward the seal.
Maier addresses the flow of a lubricant toward a seal in U.S. Pat. No. 5,322,298 entitled Shaft Seal. Specifically, Maier discloses a windback with first and second annular grooves disposed along the bore of a seal element. The annular grooves are perpendicular to the plane of the shaft and are spaced such that the first annular groove defines a distal end of the windback relative to the lubricant sump and the second annular groove is at a proximal end of the windback relative to the lubricant sump. Threads extend along the windback bore at an oblique angle, relative to the longitudinal axis of the shaft, between the first and second annular grooves. In operation, lubricant drops entering between the windback and the shaft are deposited into the oblique threads by the windage drag. The shearing stress caused by the rotation of the shaft forces the fluid to move along the oblique threads and into the annular grooves. However, there is no leak-off slot to release buildup of fluid from the annular grooves. As such, fluid can accumulate within the grooves thereby decreasing the pumping efficiency of the windage and frustrating reintroduction of lubricant into the space between the shaft and the windage.
McNickle addresses the flow of a lubricant toward a seal in U.S. Pat. No. 5,503,407 entitled Windbacks for Rotating Shafts. Specifically, McNickle discloses a windback as part of a circumferential seal assembly within a lubricant sump. The windback includes an internal helical thread machined on the bore of the housing, wherein, a shaft passes through and rotates within the bore. When the shaft rotates, windage results in a clearance between the windback and the shaft. Lubricant droplets entering between the windback and shaft clearance from the lubricant side are, thereby, forced on the helical thread due to the effect of windage drag. The lubricant droplets are further forced by the shearing stress on the thread surface to move circumferentially along the helical thread to a single windback bleed-off slot wherein the lubricant is returned to the lubricant sump. The helical thread requires the lubricant droplets to travel several times around the circumferential distance defined by the helical thread to the singular leak-off slot before dropping into the lubricant side. Ultimately, the circumferential distance around the windback helical thread combined with only one leak-off slot decreases the ability of the helical thread to efficiently pump the lubricant back into the sump. The additional distance and single leak-off slot also increases the risk that the windback will clog or back up with lubricant, reducing efficiency of the helical thread.
Roche et al. addresses the flow of a lubricant toward a seal in U.S. patent Ser. No. 12/125,133 entitled Windback Device. Specifically, Roche et al. discloses a windback including an annular collar extending from a seal housing and substantially surrounding a shaft. Along an inner face of the collar are a series of channels that are adapted to form between one or more inclined threads of either the same or varying dimensions. The threads preferably extend across the length of the collar at an oblique angle, relative to the axis of the runner. At an end of each thread, proximal to the lubricant sump and distal to the seal housing, is a leak-off slot leading to the lubricant sump. The windback is positioned over the shaft such that the collar substantially surrounds the shaft, affording the windback optimal radial clearance as respecting the distance between the ridges and the shaft. Specifically, the radial clearance is such that the inclined threads receive lubricant deposited thereon by windage resulting from rotation of the shaft. The radial clearance allows shear forces generated by rotation of the shaft to move the lubricant along the threads and through the leak-off slots of each thread. In operation, lubricant within the lubricant sump travels into the radial clearance between the shaft and the windback. Rotation of the shaft creates windage that splashes the lubricant into the threads of the windback. Sheer forces created by rotation of the shaft urge the lubricant along the threads such that the lubricant moves away from the seal and the seal housing, through the leak-off slots. Accordingly, the windback returns the lubricant back to the lubricant sump and prevents the lubricant from contacting the seal.
The devices described by Maier, McNickle, and Roche et al. employ various mechanisms to redirect fluid into a sump after entering the space between a windback and a shaft. However, such mechanisms alone are not always sufficient to prevent oil from reaching a seal and thereafter leaking into the gas side of a turbine engine.
Accordingly, what is required is a windback device for a circumferential seal capable of redirecting fluid into a sump after entering the space between a windback and a runner.